The present invention generally relates to techniques and apparatus for optimizing the use of frequency spectrum allocated for transmission of information and, more particularly, to a technique and apparatus for limiting to high and low values the luminance bandwidths of alternate fields of a television picture such that signals televised from two different sources to produce two different pictures, respectively, can occupy the same portion of channel spectrum with minimal visible artifacts.
The general problem solved by the invention is to transmit two television programs in the six megahertz channel space normally allotted for one program and allow reception of either program at each of a plurality of locations where the programs are received. This is particularly desirable in cable television systems in which a limited number of channels are available for transmitting television programs to subscribers. Several methods for achieving this improvement in spectrum utilization have been implemented in the prior art; however, each of the prior art methods have resulted in problems of performance and/or cost.
Typically, prior art systems used to process television signals have used a technique of sending one horizontal line at high bandwidth followed by limited bandwidth information which, in combination with the first line, can be used to form the second horizontal line. These techniques invariably generate visible artifacts in vertical lines that show up as alternate wide and narrow portions known as "zippering" or a stair-step pattern on slant lines known as "jaggies".
A different approach is taken in the chrominance time-compressed, luminance bandwidth-reduced television system disclosed in copending patent application Ser. No. 06/697,535. In that system, the chrominance information is transmitted in time-compressed from during the inactive picture time. By "inactive picture time" what is meant is the normal horizontal retrace time plus the time of the nonvisible overscan of a line. In the NTSC composite video signal, for example, this is the time period during which the synchronizing signal is transmitted. In order to provide room for the time-compressed chrominance information, the synchronizing signal is narrowed. The synchronizing signal may precede or follow the timecompressed chrominance information, and the polarity of the synchronizing signal may be reversed to allow an increase in the dynamic range of the composite signal. Chrominance information for a given program is sent once every other line with the other program chrominance information occupying the unused line position. Luminance information for both programs is sent during the active video time for each line. By "active video time" what is meant is that time period of the composite video signal during which a visible image is produced. The way in which luminance information for both programs is sent simultaneously is by modulating one program on the main carrier and modulating the second program on a subcarrier approximately 4.5 MHz removed from the main carrier.
The television system disclosed in copending patent application Ser. No. 06/697,535 would exhibit a deficiency, were it not for the present invention which is employed in that system. Specifically, the luminance bandwidth of a given television program, under the NTSC standards, is limited to 2.25 MHz. This results in a "soft" picture; i.e., one in which the edges of images viewed on the screen are not sharply defined. Although the picture can be enhanced by the use of peaking, as disclosed in copending patent application Ser. No. 06/697,600, this enhancement, by itself, would nevertheless be judged to result in pictures somewhat softer than NTSC standard pictures.